Research Of Self-defocusing And Regulation Mechanism Of The Third-order Nonlinear Effects Based On Photonic Crystals

Nonlinear optical effects are critical for modern optical applications such as laser frequency conversion,ultrashort pulse generation,and all-optical signal processing and ultrafast optical switching.However,conventional non-linearity of optical materials is very small and slow response.In order to better improve and expand these applications,non-linear optical research scholars have been dreaming of a new type of non-linear medium both with large non-linear coefficient and ultra-fast response speed.Photonic crystals,metamaterials and other artificial electromagnetic medium,make the scholars dream come true to provide a possible technical path.Artificial electromagnetic medium owns a special properties which natural material does not have as a electromagnetic artificial microstructure materials,has been the focus of scientific research personnel research in the past 20 years.Contrast to metamaterial,photonic crystals do not contain metal components and thus have lower losses.We studied on the third-order nonlinear effects of two-dimensional photonic crystals and the mechanism of self-defocusing and nonlinear effects in this photonic crystals.The background,significance,development status and related theories of photonic crystal control nonlinearity are reviewed in this paper.The main work and innovation are as follows:Firstly,the method of extracting the equivalent nonlinear refractive index of artificial microstructure based on finite difference time domain method is established based on the spectral broadening caused by self-phase modulation.The finite difference time domain method is used to simulate the transmission of the chirped pulse in the nonlinear artificial microstructure.The equivalent nonlinear refractive index coefficient of the artificial microstructure is obtained by matching the outgoing pulse spectrum and the theoretical calculation spectrum.Secondly,it is proved that the field effect and the slow light effect are the physical instincts to control the nonlinear effect of the photonic crystal from the perspective of the equivalent nonlinear refractive index coefficient of the photonic crystal.In the first band of the two-dimensional photonic crystal,we first modify the equivalent dielectric constant of the photonic crystal from the average field in the original electric field.Secondly,the theoretical expression of the equivalent nonlinear refractive index coefficient of the photonic crystal is obtained by introducing the slow light enhancement factor.Finally,we use the finite difference time domain method to extract the equivalent nonlinear refractive index coefficient of the photonic crystal,and find it in agreement with the theoretical calculation results.Thirdly,the simulation proves the controllable self-focusing effect in the negative refractive index photonic crystal.We first insert a silicon rod into a two-dimensional photonic crystal in a self-focusing non-linear dielectric carbon disulfide.Secondly,the equivalent nonlinear refractive index coefficient of the second band,ie,the negative refractive index frequency band,is extracted by the finite difference time domain method,and the coefficient is found to be positive.Finally,we use the time-domain finite difference method to simulate the transmission of the gaussian beam in the nonlinear photonic crystal.It is found that the convergent gaussian beam has self-defocusing in the photonic crystal,and the divergent gaussian beam converges in the photonic crystal.But the focal spot is bigger than the waist of incident divergence gaussian beam.